As shown in FIG. 1, an Media Gateway Controller (MGC) and an Media Gateway (MG) are key components in an Next Generation Network (NGN). The MGC provides call control function, while the MG provides service bearing function, thereby implementing separation between call control plane and service bearing plane, and thus sharing network resources efficiently, simplifying upgrade of equipments and service expansion, and greatly reducing costs for development and maintenance.
Media Gateway Control Protocol is the main protocol between MGs and MGCs; presently, two protocols are widely used, i.e., H.248/MeGaCo and MGCP. In H.248 protocol, for example, the resources in MGs are abstracted as terminations. The terminations are categorized into physical terminations and ephemeral terminations; the former represents some semi-permanent physical entities, such as TDM channels; the latter represents some public resources that are obtained on request temporarily and then released after use, such as RTP streams. Any combination among terminations is abstracted as context. A context may include several terminations and thereby the relationship among terminations can be topologically described.
Based on above model, call switching can be regarded as an operation on terminations and contexts. Such operations are accomplished by means of command requests and responses between MGCs and MGs. Such commands carry parameters also referred as descriptors. The descriptors are categorized into property, signal, event, and statistic. Parameters with service correlation are aggregated into a package logically.
Signals are used by an MGC to instruct an MG to perform operations of corresponding resources, e.g., send dial tone, ring-back tone, or busy tone to a subscriber. Events are used by an MGC to instruct an MG to monitor corresponding status, e.g., monitor whether a subscriber hangs off, hangs up, flashes the hook, and dials a number, etc. Events are usually triggered by status change and may result in corresponding responses, e.g., the dial tone is sent to the subscriber only when the subscriber hangs off.
However, in the present definition in H.248 protocol, events have to be issued by an MGC to an MG before the MG can start monitoring the corresponding status change. The mechanism is suitable in most cases when the MGC is viewed as a controller while the MG is viewed as a controlled entity. However, in some special cases, the mechanism has some disadvantages. For instance, after the MG is successfully registered to the MGC, the MGC's activation status must be monitored immediately, which can be implemented by reporting an “Inactivity” Timeout event by the MG to trigger the MGC to feed back a response. However, in the existing H.248 protocol, if the MGC doesn't issue the event to the MG due to some causes (e.g., MGC is not configured, sudden fault, or transmission failure), the MG will not monitor the MGC's activation status. As a consequence, the MG can operate abnormally because it has lost the control of the MGC. Therefore, a breakdown can occur in a part of the network system in certain cases, thereby degrading the stability of the entire network system.